This invention relates to fan and heat sink assemblies as might be used for heat sinking semiconductor devices. Usually a fan is mounted so that it blows air over a heat sink assembly. The rate at which heat can be removed from the heat sink is a function of the heat sink surface area, the temperature difference from the heat sink to the air, and the velocity of the air. If the air has high velocity and is turbulent, the heat sink can be relatively smaller, but there are limitations to the extent that this is practical. It takes a large, powerful fan to move air with a high velocity. Such a fan would be expensive, would take substantial power to operate, and would be large and noisy.
This invention teaches that there is a region within a ducted fan where the air naturally has a very high velocity and is turbulent the inside surface of the fan duct in the area swept by the fan blades. The fan blades move the air very rapidly, and also generate blade tip vortices and wake turbulence, so heat flow into the air stream is greatly enhanced. By mounting semiconductors or other devices needing heat sinking on the outside of the fan duct and in good thermal contact with it, a superior heat sink is achieved.
This invention further teaches several modifications to the inside surface of the fan duct and/or the fan blades to further enhance heat flow through the fan duct as a trade-off with fan performance as an axial flow device. In some embodiments, axial air flow from the fan assembly is entirely eliminated in favor of maximally enhancing heat transfer within the fan assembly.
Often the amount of air needed to transport the waste heat away from a heat sink is small compared to the amount of air that is needed to sustain sufficient air velocity for good heat transfer. Several embodiments of the invention teach that much of the air flow can be recirculated within the fan assembly with heat sink to provide high air flow internally across the heat transfer surfaces. The inlet and exit air flow is then quite low, which will make operation much quieter, and greatly reduce the design requirements of accessories such as filters and finger guards which may be required on the inlet and/or outlet.
In a vane axial fan, the inlet and/or outlet vanes can also serve as heat transfer surfaces. In a multi-stage fan, the baffles or flow straighteners between the fan stages can also serve as heat transfer surfaces. These features can be optimized for heat transfer as a trade off with maximum fan performance in the usual sense.